This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-017216, filed Jan. 26, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to an over current protection circuit of a gate voltage controlled type semiconductor switching device, and more particularly to a control circuit for controlling a semiconductor switching device on the basis of the detected current result, the circuit being used in a semiconductor switching device such as, for example, an IGBT (insulated gate bipolar transistor), a power MOSFET (metal-oxide-silicon field effect transistor) or the like.
The IGBT which is a gate voltage controlled type semiconductor switching device includes an IGBT (a sense IGBT) having a current detection terminal through which a detection current flows in accordance with a main current (a load current).
FIG. 5 shows a basic circuit of a sense IGBT, and FIG. 6 shows a conventional over current protection circuit of the sense IGBT.
In the circuit shown in FIG. 5, reference numeral 10 denotes the sense IGBT. The IGBT comprises a plurality of unit cells having the same structure provided on the same semiconductor chip are divided to be M:1 on the side of the main current element 10a and on the side of detection current element 10b. These unit cells 10a and 10b are connected in parallel to each other.
The gate of the main current element 10a and the gate of the detection current element 10b are connected to the gate terminal 11, and the collector of the main current element 10a and the collector of the detection current element 10b are connected to the collector terminal 12. As a consequence, the ratio of a main current IMAIN flowing through the main current element 10a and a detected current ISENS flowing through the detection current element 10b is equal to the division ratio of the unit cells. In the circuits shown in FIGS. 5 and 6, a current IC denotes a sum of the main current IMAIN and the detected current ISENS.
For over current protection of the IGBT 10, in this embodiment, as shown in FIG. 6, a resistor 15 for current detection is connected between the emitter terminal 13 on the side of the main current element 10a and a current detection terminal (a sense terminal) 14 which is an emitter terminal on the side of the detection current element 10b. Further, an NMOS transistor 50 is connected between the gate of the IGBT 10 and the emitter terminal 13. The NMOS transistor 50 has a drain connected to the gate of the sense IGBT 10, an source connected to one terminal of the current detection resistor 15, a gate connected to the other terminal of the current detection resistor 15, so that a voltage drop (the detected voltage) across the current detection resistor 15 is applied to the gate of the NMOS transistor 50.
In the over current protection circuit shown in FIG. 6, the main current element 10a and the detection current element 10b are shown as a single combined element 10. This is only for the simplicity of the drawing. The main current element 10a and the detection current element 10b are in a form of separate components in the actual circuit structure.
With this circuit structure, it is possible by the NMOS transistor 50 to detect the time when the detected voltage reaches a set value for an over current cut-off. That is, when an over current flows through the sense IGBT 10, the NMOS transistor 50 is turned on, so that the gate voltage of the sense IGBT 10 is decreased to 0V and thus the sense IGBT 10 is turned off.
In the over current protection circuit shown in FIG. 6, a reference numeral 16 denotes a gate resistor connected in series to the gate of the sense IGBT. A reference numeral 17 denotes a diode for a reverse bias prevention connected in a forward direction between a gate of the sense IGBT 10 and a drain of an NMOS transistor 50.
For the current detection resistor 15, the NMOS transistor 50, the gate resistor 16, the diode 17 and the capacitor element 18, individual parts separate from the sense IGBT 10 are used. Alternatively, these circuit components and the IGBT 10 are formed on the same semiconductor chip. Further alternatively, these circuit components are formed on a semiconductor chip for controlling the sense IGBT, separate from the semiconductor chip on which the sense IGBT 10 is formed.
FIG. 7 shows waveforms used for explaining time transition of the over current protection operation of the sense IGBT in the over current protection circuit of FIG. 6.
The gate voltage VG is constant until the detection of the over current of the sense IGBT. At the time of the detection of an over current of the sense IGBT (i.e., when the detected voltage VSENS reaches the over current cut-off set value), the NMOS transistor 50 is turned on, the gate voltage of the sense IGBT 10 is decreased to 0V and the sense IGBT is turned off to cut the main current IMAIN off. As a consequence, it is possible to realize a protection function for preventing the breakdown of the sense IGBT 10 due to the over current.
However, since a voltage drop (the detected voltage VSENS) is generated between an emitter terminal on the side of the main current element 10a and a current detection terminal 14 on the side of the detection current element 10b, the voltage VCE applied between the collector and the emitter of the main current element 10a is different from the voltage VCS applied between the collector and the emitter of the detection current element 10b by the detected voltage VSENS.
That is, the following equation is established.
VCS=VCExe2x88x92VSENS 
VCS: a voltage between the collector and the current of the detection current element 10b 
VCE: a voltage between the collector and the emitter of the main current element 10a 
VSENS: detected voltage
Consequently, the current which actually flows through the detection current element 10b is smaller than the current of the design value by the division ratio of the unit cells. As the characteristic of the sense IGBT has become improved and the on-voltage has become decreased, the difference between the current actually flows and the current of the design value greatly influences on a difference between a voltage between the collector and the emitter of the main current element 10a and a voltage between the collector and the emitter of the detection current element 10b, and as a result a sufficient detection current ISENS cannot be obtained.
FIG. 8 is a characteristic graph showing relational curves between the detected voltage VSENS in the over current protection circuit shown in FIG. 6 and the main current IMAIN of the sense IGBT.
As can be seen from this characteristic graph, before the characteristic of the sense IGBT has been improved and the on-voltage has been decreased, the correlation between a detected voltage VSENS and the main current IMAIN shows linearity as shown by the characteristic curve I. However, recently, since the characteristic of the sense IGBT has been greatly improved and thus the on-voltage has been largely decreased, the linearity is lost and an abrupt curve as shown by the characteristic curve II is generated.
As a consequence, a variation in the main current IMAIN becomes large with respect to the variation in the detected voltage VSENS and the cut-off level of the over current largely varies, and a sufficient protection function cannot be realized. In the worst case, it is impossible to prevent the breakdown of the sense IGBT 10.
As a countermeasure against such a disadvantage, it is considered that the detected voltage VSENS is lowered by decreasing the value of the current detection resistor 15 in the over current protection circuit, and a difference between a voltage between the collector and the emitter of the main current element 10a and a voltage between the collector and the emitter of the detection current element 10b of the sense IGBT is decreased by lowering the over current cut-off set value. However, with this countermeasure, an error in operation is likely to be generated due to noises.
As has been described above, the conventional over current protection circuit of the sense IGBT has a disadvantage in that the variation in the main current becomes large with respect to the variation in the detected voltage, and the cut-off level of the over current also largely varies and a sufficient protection function cannot be obtained. In the worst cases, it is impossible to prevent the breakdown of the sense IGBT.
The present invention has been made to solve the above mentioned disadvantages, and an object of the invention is to provide an over current protection circuit of a gate voltage controlled type semiconductor switching device which is capable of suppressing a variation in a main current with respect to a variation in a detected voltage, suppressing a variation in the cut-off level of the over current, and thus realizing a sufficient protection function of the semiconductor switching device, by allowing the circuit to be provided with a characteristic that a variation in the main current with respect to a variation in the detected voltage of the current detection resistor connected to a current detection terminal of the gate voltage controlled type semiconductor switching device is gentle in the region where the semiconductor switching device is turned off.
In order to achieve the objection, an over current protection circuit of a semiconductor switching device according to a first aspect of the present invention comprises: a gate voltage controlled type semiconductor switching device having a main current element with a first current output terminal and a detection current element with a second current output terminal; a resistor connected between the first current output terminal and the second current output terminal, for detecting a voltage; and a control circuit connected to a gate terminal of the semiconductor switching device and the resistor, for decreasing a gate voltage applied to the semiconductor switching device when the gate voltage exceeds a first set value larger than the detected voltage detected by the resistor, and cutting off the gate voltage applied to the semiconductor switching device when the gate voltage reaches a predetermined voltage set for a predetermined over current cut-off value.
In the over current protection circuit of a semiconductor switching device according to the first aspect of the present invention, when the detected voltage exceeds a second set value larger than the first set value, the control circuit may decrease the gate voltage applied to the semiconductor switching device in a characteristic curve different from that when the gate voltage exceeds the first set value.
In the over current protection circuit of the semiconductor switching device according to the first aspect of the present invention, the control circuit may comprise a bipolar transistor. The bipolar transistor may be an NPN transistor. The resistor and the bipolar transistor may be formed on the same semiconductor chip. The resistor and the NPN transistor may be formed on the same semiconductor chip.
In the over current protection circuit of the semiconductor switching device according to the first aspect of the present invention, the gate voltage controlled type semiconductor switching device may be an insulation gate bipolar transistor.
In the over current protection circuit of the semiconductor switching device according to the first aspect of the present invention, the gate voltage controlled type semiconductor switching device may be an insulated gate field effect transistor.
In the over current protection circuit of the semiconductor switching device according to the first aspect of the present invention, the control circuit may comprise a bipolar transistor one output terminal of which is connected to the first current output terminal, a diode connected between the other output terminal of the bipolar transistor and the gate terminal of the semiconductor switching device, and a capacitor connected between a base terminal of the bipolar transistor and the other output terminal of the bipolar transistor.
In the over current protection circuit of the semiconductor switching device according to the first aspect of the present invention, the over current protection circuit may further comprise a gate resistor connected in series with the gate terminal of the semiconductor switching device.
An over current protection circuit of a semiconductor switching device according to a second aspect of the present invention comprises: a gate voltage controlled type semiconductor switching device having a main current element with a first current output terminal and a detection current element with a second current output terminal; a resistor connected between the first current output terminal and the second current output terminal, for detecting a voltage; and a control circuit connected to a gate terminal of the semiconductor switching device and the resistor, for decreasing a gate voltage applied to the semiconductor switching device in a characteristic curve when the gate voltage exceeds a first set value larger than the detected voltage detected by the resistor, decreasing the gate voltage in a different characteristic curve when the gate voltage exceeds a second set value larger than the first set value, successively decreasing the gate voltage in further different characteristic curves when the gate voltage successively exceeds further higher set values, and cutting off the gate voltage when the gate voltage reaches a predetermined voltage set for a predetermined over current cut-off value.
In the over current protection circuit of the semiconductor switching device according to the second aspect of the present invention, the control circuit may comprise a bipolar transistor. The bipolar transistor may be an NPN transistor. The resistor and the bipolar transistor may be formed on the same semiconductor chip. The resistor and the NPN transistor may be formed on the same semiconductor chip.
In the over current protection circuit of the semiconductor switching device according to the second aspect of the present invention, the gate voltage controlled type semiconductor switching device may be an insulated gate bipolar transistor.
In the over current protection circuit of the semiconductor switching device according to the second aspect of the present invention, the gate voltage controlled type semiconductor switching device may be an insulated gate field effect transistor.
In the over current protection circuit of the semiconductor switching device according to the second aspect of the present invention, the control circuit may comprise a bipolar transistor one output terminal of which is connected to the first current output terminal, a diode connected between the other output terminal of the bipolar transistor and the gate terminal of the semiconductor switching device, and a capacitor connected between a base terminal of the bipolar transistor and the other output terminal of the bipolar transistor.
In the over current protection circuit of the semiconductor switching device according to the second aspect of the present invention, the over current protection circuit may further comprise a gate resistor connected in series with the gate terminal of the semiconductor switching device.
An over current protection circuit of a semiconductor switching device according to a third aspect of the present invention comprises: a gate voltage controlled type semiconductor switching device having a main current element with a first current output terminal through which a main current flows, and a detection current element with a second current output terminal through which a detection current flows in correspondence to the main current; a resistor for current detection connected between the current output terminal of the main current element and the current output terminal of the detection current element for generating a voltage drop generated by the current flowing through the detection current element; and a control circuit connected to a gate terminal of the semiconductor switching device and the resistor, for decreasing a gate voltage applied to the semiconductor switching device in a first characteristic curve when the gate voltage exceeds a first set value larger than the detected voltage at the time of the detection of the rated current of the semiconductor switching device, and cutting off the main current when the gate voltage reaches a predetermined voltage set for a predetermined current value larger than a current value set for the first set voltage value.
In the over current protection circuit of a semiconductor switching device according to the third aspect of the present invention, when the detected voltage exceeds a second set value larger than the first set value, the control circuit may decrease the gate voltage in a second characteristic curve different from the first characteristic curve.
In the over current protection circuit of the semiconductor switching device according to the third aspect of the present invention, the control circuit may comprise an NPN transistor. The current detection resistor and the NPN transistor may be formed on the same semiconductor chip.
In the over current protection circuit of the semiconductor switching device according to the third aspect of the present invention, the gate voltage controlled type semiconductor switching device may be an insulation gate bipolar transistor.
In the over current protection circuit of the semiconductor switching device according to the third aspect of the present invention, the gate voltage controlled type semiconductor switching device may be an insulation gate field effect transistor.
An over current protection circuit of a semiconductor switching device according to a fourth aspect of the present invention comprises: a gate voltage controlled type semiconductor switching device having a main current element with a first current output terminal through which a main current flows, and a detection current element with a second current output terminal through which a detection current flows in correspondence to the main current; a resistor for current detection connected between the current output terminal of the main current element and the current output terminal of the detection current element for generating a voltage drop generated by the current flowing through the detection current element; and a control circuit connected to a gate terminal of the semiconductor switching device and the resistor, for decreasing a gate voltage applied to the semiconductor switching device in a first characteristic curve when the gate voltage exceeds a first set value larger than the detected voltage at the time of the detection of the rated current of the semiconductor switching device, decreasing the gate voltage in a different characteristic curve when the gate voltage exceeds a second set value larger than the first set value, successively decreasing the gate voltage in further different characteristic curves when the gate voltage successively exceeds further higher set values, and cutting off the main current when the gate voltage reaches a predetermined voltage set for a predetermined over current cut-off value.
In the over current protection circuit of the semiconductor switching device according to the fourth aspect of the present invention, the control circuit may comprise an NPN transistor. The current detection resistor and the NPN transistor may be formed on the same semiconductor chip.
In the over current protection circuit of the semiconductor switching device according to the fourth aspect of the present invention, the gate voltage controlled type semiconductor switching device may be an insulation gate bipolar transistor.
In the over current protection circuit of the semiconductor switching device according to the fourth aspect of the present invention, the gate voltage controlled type semiconductor switching device may be an insulation gate field effect transistor.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.